The present invention relates to a metal collector foil for use in an electric double layer capacitor, a method of producing the metal collector foil, and an electric double layer capacitor using the metal collector foil.
Electric double layer capacitors are known heretofore. Japanese Patent Laid-open Publication (JP-A1) No. 11-283871 discloses an invention that focuses in particular on the strength of a metal collector foil for such electric double layer capacitors. The metal collector foil is made of aluminum and has a roughened surface layer of 1 to 5 xcexcm thick. It is stated in this publication that as for roughened surface layers having thicknesses greater than 5 xcexcm, no further improvement in the bonding strength is achieved; The larger the thickness of the roughened surface layer, the smaller the strength of the aluminum foil; Especially, a 2-4 xcexcm thick roughened surface is preferable.
Based on the disclosure of JP-A1-11-283871, many sample electric double layer capacitors were produced by the present inventors for evaluation. The produced samples indicated that the metal collector foil was satisfactory in terms of strength, but due to an insufficient bonding strength relative to a sheet-like electrode material, electrode separation did occur during operation of the electric double layer capacitors, which significantly lowered the electricity storage capability of the capacitor. In this regard, it may be considered that the metal collector foil of the afore-said Japanese publication was designed mainly for keeping a desired mechanical strength, and for that reason, formation of an etched rough surface layer to increase the bonding strength was insufficiently addressed.
It is accordingly an object of the present invention to provide a metal collector foil for use in an electric double layer capacitor, which is capable of providing a sufficient bonding strength relative to an electrode material while keeping a desired mechanical strength.
Another object of the present invention is to provide a method of producing such metal collector foil.
A further object of the present invention is to provide an electric double layer capacitor using the same metal collector foil.
According to a first aspect of the present invention, there is provided a metal collector foil for an electric double layer capacitor, which comprises an etched metal collector foil having etched upper and lower surface layers and an unetched central layer disposed between the etched upper and lower surface layers. The etched upper and lower surface layers have a total thickness sufficient to provide the metal collector foil with a capacitance per unit area that corresponds to a capacitance value obtained when the etched metal collector foil is subjected to an anodic formation process with application of a withstanding voltage of 65.5 volts, the capacitance value being not less than 1.7 xcexcF/cm2. The unetched central layer has a thickness sufficient to provide the metal collector foil with a tensile strength not less than 9,000 N/cm2.
Since the unetched central layer has a thickness sufficient to provide the metal collector foil with a tensile strength not less than 9,000 N/cm2, it is possible to prevent the metal collector foil from being broken or ruptured during the winding process. Additionally, since the etched upper and lower surface layers have a total thickness sufficient to provide the metal collector foil with a capacitance per unit area that corresponds to a capacitance value obtained when the etched metal collector foil is subjected to an anodic formation process with application of a withstanding voltage of 65.5 volts, and since the capacitance value is not less than 1.7 xcexcF/cm2, a sufficiently high bonding strength can be provided between the metal collector foil and the electrode material. This ensures that an electric double layer capacitor in which the metal collector foil is used can operate stably and reliably without causing accidental electrode separation.
The metal collector foil may comprise an aluminum foil having a purity of at least 99.8% and an overall thickness of approximately 39 xcexcm, in which instance the total thickness of the etched upper and lower surface layers is in a range of 22 to 30 xcexcm, and the thickness of the unetched central layer is in a range of 9 to 17 xcexcm.
According to a second aspect of the invention, there is provided a method of producing a metal collector foil for use in an electric double layer capacitor, which comprises the steps of: preparing a plain metal foil; and etching the metal foil in a chloride solution to dissolve the surface of the metal foil until etched upper and lower surface layers of the metal foil have a total thickness sufficient to provide the metal foil with a capacitance per unit area that corresponds to a capacitance value obtained when the etched metal collector foil is subjected to an anodic formation process with application of a withstanding voltage of 65.5 volts, the capacitance value being not less than 1.7 xcexcF/cm2, and an unetched central layer of the metal foil has a thickness sufficient to provide the metal foil with a tensile strength not less than 9,000 N/cm2.
In one preferred form of the invention, the metal collector foil comprises an aluminum foil having a purity of at least 99.8% and an overall thickness of approximately 39 xcexcm. In this case, the etching is carried out to the extent that the total thickness of the etched upper and lower surface layers is in a range of 22 to 30 xcexcm, and the thickness of the unetched central layer is in a range of 9 to 17 xcexcm.
According to a third aspect of the present invention, there is provided an electric double layer capacitor comprising a positive electrode and a negative electrode each having a metal collector foil and an electrode material formed mainly from activated carbon and bonded to both opposite surfaces of the metal foil, a dielectric separator disposed between the positive and negative electrodes, and a liquid electrolyte impregnated in the electrode material to enable charging and discharging of the electric double layer capacitor. The metal collector foil has etched upper and lower surface layers and an unetched central layer disposed between the etched upper and lower surface layers. The etched upper and lower surface layers have a total thickness sufficient to provide the metal collector foil with a capacitance per unit area that corresponds to a capacitance value obtained when the etched metal collector foil is subjected to an anodic formation process with application of a withstanding voltage of 65.5 volts, the capacitance value being not less than 1.71 xcexcF/cm2, and the unetched central layer has a thickness sufficient to provide the metal collector foil with a tensile strength not less than 9,000 N/cm2.